As is well known, an optical signal may have two orthogonal polarization states, each of which may have different properties. Sometimes such polarization states are intentionally introduced, such as in creating a polarization-multiplexed signal in which the two orthogonal polarization states of the optical carrier are arranged so that each carries different data in order to double the spectral efficiency. Such a polarization-multiplexed signal has two so-called “generic” polarization components, each of which carries a single data modulation. Note that by a generic polarization component it is generally intended the signal at the point at which the modulation of that polarization component is completed. It should be appreciated that each generic polarization component may initially, or otherwise, exist separate from the other generic polarization component with which it is later combined. It should also be appreciated that the phase of the generic need not be constant.
Polarization-division-multiplexed optical communication systems using digital coherent detection are promising candidates for use in high speed optical networks.
Unfortunately, the polarization orientations of the generic signal components are generally changed by the birefringence of the fiber, and possibly other fiber properties, during the passage of the signal over the optical path. Such changes may be time varying because at least the fiber birefringence is typically a function of various factors such as ambient temperature, mechanical stress, and so forth, which may vary over time and be different at various points of the transmission path. As a result, the polarization orientation of each of the generic signal components is generally unknown at the receiver.
Sometimes, undesirably, the fiber birefringence is so large that polarization-mode dispersion (PMD) is caused, i.e., a generic optical signal component is decomposed into two orthogonal polarization components along the two principal state of polarization (PSP) axes of the fiber, along one of which the light travels at its fastest speed through the fiber and along the other of which the light travels at its slowest speed through the fiber. In such a case, not only may the phase relationship between the two polarization components be time varying, but also each of the two orthogonal polarization components may arrive at the receiver at different times due to the PMD-induced differential group delay (DGD) between the two PSP axes. Note that, actually, as suggested above, each small section of the fiber behaves as if it is its own mini fiber that introduces its own DGD between the two PSP axes. Thus, for a particular fiber or optical link, PMD is a stochastic effect, and the PMD-induced DGD may also be time varying.
Optical communication systems also suffer from polarization dependent loss (PDL). PDL mainly comes from optical components such as couplers, isolators and circulators, in which insertion loss is dependent on polarization states of input signals. PDL causes the fluctuation of optical signal-to-noise-ratio (OSNR) and performance differences between the two generic polarization components. PDL is a stochastic phenomenon and PDL-induced penalties may also be time varying.
Other linear effects distort optical signals transmitted over optical fibers. Such effects include chromatic dispersion (CD) which is a deterministic distortion given by the design of the optical fiber. CD leads to a frequency dependence of the optical phase and its effect on transmitted signal scales quadratically with the bandwidth consumption or equivalently the data rate. Optical compensation methods and electrical compensation methods are typically employed to reduce signal distortion that arises due to CD or PMD in direct detection systems and coherent detection systems, respectively.
In prior art polarization-division-multiplexed optical coherent communication systems, transmission impairments, such as chromatic dispersion, polarization-mode dispersion, and polarization dependent loss, may be compensated for electronically using digital signal processing, and polarization demultiplexing of the generic polarizations may also performed in the electrical domain by digital signal processing. Unfortunately, such prior art systems suffer from various disadvantages. For example, Digital Filters For Coherent Optical Receivers By Savory, published in Optics Express vol. 16, No. 2, 2008 pp. 804-817, pointed out that the prior art systems suffer from the so-called “singularity problem”, which means that the output two polarization tributaries tend to converge to the same source. The same problem is also encountered in the system described in Initial tap setup of Constant Modulus Algorithm For Polarization De-Multiplexing In Optical Coherent Receivers by Lin et al. published in OSA/OFC/NFOEC 2009 as paper number OMT2.